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The 3 Most Common Steel Hardening Heat Treatments & Applications

Heat Treated CNC Machined Steel and Stainless Steel Parts Manufactured by Challenge Engineering

Heat treatment refers to the process of heating or cooling materials such as steel or glass, in order to alter their properties in some way. The result of heat treating is usually a hardening or softening of the material.

This article will concentrate on the industrial hardening of steel, which is carried out using one of three main techniques.

We’ll look at each individually, what they involve, and some of the applications of the hardened steel that is produced in each case.

1. Case Hardening

As the title of the process suggests, this hardening technique is used to strengthen by creating a tough outer case on the surface of the steel, while keeping a normal level of hardness and ductility in the interior. This is achieved by adding extra carbon or nitrogen to the outer layer of the steel (or iron).

The process is sometimes referred to as case carburisation. The steel is usually a soft, low-carbon one that needs strengthening.

Several industrial processes can be used to case harden steel, the main one being flame or induction hardening. This involves heating the surface to an extremely high temperature, then cooling it rapidly which creates an outer case of martensite, providing the needed carbon and hardness.

Case hardening increases the lifespan of the steel and improves resistance to fatigue. For these reasons, it is often used to strengthen:

  • cooking utensils,

  • automotive parts, and

  • tools that receive a lot of wear.

The downside of case hardening is that it can make the surface more brittle, so it isn’t suitable for objects that will slide against abrasive surfaces.

To counteract the brittleness, sometimes the metal is quenched after heating.Quenching is the soaking of a metal at a high temperature, above the recrystallization phase, followed by a rapid cooling process to obtain certain desirable material properties.

Case hardening is also sometimes referred to as face hardening, especially when applied to metals that are to be used for military-grade armour.

2. Through Hardening

Through hardening increases the hardness and strength of steel alloys throughout the metal structure, not just the exterior as with case hardening.

The process usually involves three main steps:

  1. heating the alloy

  2. quenching it in a substance such as water or oil

  3. reheating (or tempering) the metal to make the surface less brittle.

The resulting steel alloy is strong, hard, wear-resistant and less ductile than in its normal state. The main applications are:

  • engineering parts that need to bear heavy loads such as brackets, nuts, and bolts, hooks, nails, screws, etc.

  • hand tools such as hammers, screwdrivers, etc.

  • springs, bearings, axles, and other load-bearing components.

The most common steel alloys used for through hardening are 4140 and 4340 high tensile alloys as they yield an optimal amount of strength and hardness.

3. Flame Hardening

Flame hardening is a process that hardens the outer layer of a steel alloy. It involves heating the surface to high temperatures under a flame.

When it reaches its critical temperature, the alloy is quenched in water or a synthetic substance to quickly harden the surface to a depth between 1 and 10mm.

The steel used must have a carbon content of more than 0.35% for the process to work. The resulting metal has high strength and wear resistance, but maintains ductility in its core.

This technique requires a specialist furnace-like machine to reach the high temperatures and is carried out in an industrial setting.

Using this method, flat surfaces of sheet metal can be hardened, making it ideal for items such as:

  • kitchen knives,

  • wear plates, and

  • flat tools such as hand saws.

Another closely related method of hardening is known as induction hardening, which involves heating and strengthening the steel by running an alternating current through the structure, rather than applying a flame.

In Summary

Hardening steel is a common process used to toughen many engineering items, as well as everyday products.

There are three main techniques to harden steel:

  • Case hardening – involves adding carbon through heating to strengthen the outer layer, ideal for cooking utensils and hard-wearing parts, that don’t require ductile properties.

  • Through hardening – involves heating and rapidly cooling the steel to strengthen the entire structure, ideal for fittings and hand tools

  • Flame hardening – involves heating the outer layer to create a tough outer layer of 1-10mm depth, ideal for hard-wearing, flat surfaced items and sheets such as knives and saws.

At ISO:9001 certified Challenge Engineering, we can advise you on the best type of heat treatment for your custom parts. Call (02) 9632 0010 or visit www.challengecnc.com.au, complete our Request for Quote form or Contact Form.

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